Normally, the hardware stores will sell sch 40. Ask about it before you buy. It seems that the fittings for most of those are actually rated higher than the pipe itself, so go with the pipe rating. If the '20' is your diameter, then you probably have about a 3/4" pipe.

In my sig is a link to the Hubb Library. If you follow it, you will find a portable tank calculator. In it, I've put together pipe information, which includes the type you are asking about. It shows the pressure rating, which will give you an idea.

D_Hall pretty much covered the advantages / disadvantages of it. But, for the record, I've used Sch40 galv pipe (1") with unregulated CO2 with no problems.